net: xsk: add a simple buffer reuse queue

XSK UMEM is strongly single producer single consumer so reuse of
frames is challenging.  Add a simple "stash" of FILL packets to
reuse for drivers to optionally make use of.  This is useful
when driver has to free (ndo_stop) or resize a ring with an active
AF_XDP ZC socket.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

include/net/xdp_sock.h

@@ -21,6 +21,12 @@ struct xdp_umem_page {
 	dma_addr_t dma;
 };
 
+struct xdp_umem_fq_reuse {
+	u32 nentries;
+	u32 length;
+	u64 handles[];
+};
+
 struct xdp_umem {
 	struct xsk_queue *fq;
 	struct xsk_queue *cq;
@@ -37,6 +43,7 @@ struct xdp_umem {
 	struct page **pgs;
 	u32 npgs;
 	struct net_device *dev;
+	struct xdp_umem_fq_reuse *fq_reuse;
 	u16 queue_id;
 	bool zc;
 	spinlock_t xsk_list_lock;
@@ -75,6 +82,10 @@ void xsk_umem_discard_addr(struct xdp_umem *umem);
 void xsk_umem_complete_tx(struct xdp_umem *umem, u32 nb_entries);
 bool xsk_umem_consume_tx(struct xdp_umem *umem, dma_addr_t *dma, u32 *len);
 void xsk_umem_consume_tx_done(struct xdp_umem *umem);
+struct xdp_umem_fq_reuse *xsk_reuseq_prepare(u32 nentries);
+struct xdp_umem_fq_reuse *xsk_reuseq_swap(struct xdp_umem *umem,
+					  struct xdp_umem_fq_reuse *newq);
+void xsk_reuseq_free(struct xdp_umem_fq_reuse *rq);
 
 static inline char *xdp_umem_get_data(struct xdp_umem *umem, u64 addr)
 {
@@ -85,6 +96,35 @@ static inline dma_addr_t xdp_umem_get_dma(struct xdp_umem *umem, u64 addr)
 {
 	return umem->pages[addr >> PAGE_SHIFT].dma + (addr & (PAGE_SIZE - 1));
 }
+
+/* Reuse-queue aware version of FILL queue helpers */
+static inline u64 *xsk_umem_peek_addr_rq(struct xdp_umem *umem, u64 *addr)
+{
+	struct xdp_umem_fq_reuse *rq = umem->fq_reuse;
+
+	if (!rq->length)
+		return xsk_umem_peek_addr(umem, addr);
+
+	*addr = rq->handles[rq->length - 1];
+	return addr;
+}
+
+static inline void xsk_umem_discard_addr_rq(struct xdp_umem *umem)
+{
+	struct xdp_umem_fq_reuse *rq = umem->fq_reuse;
+
+	if (!rq->length)
+		xsk_umem_discard_addr(umem);
+	else
+		rq->length--;
+}
+
+static inline void xsk_umem_fq_reuse(struct xdp_umem *umem, u64 addr)
+{
+	struct xdp_umem_fq_reuse *rq = umem->fq_reuse;
+
+	rq->handles[rq->length++] = addr;
+}
 #else
 static inline int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 {
@@ -128,6 +168,21 @@ static inline void xsk_umem_consume_tx_done(struct xdp_umem *umem)
 {
 }
 
+static inline struct xdp_umem_fq_reuse *xsk_reuseq_prepare(u32 nentries)
+{
+	return NULL;
+}
+
+static inline struct xdp_umem_fq_reuse *xsk_reuseq_swap(
+	struct xdp_umem *umem,
+	struct xdp_umem_fq_reuse *newq)
+{
+	return NULL;
+}
+static inline void xsk_reuseq_free(struct xdp_umem_fq_reuse *rq)
+{
+}
+
 static inline char *xdp_umem_get_data(struct xdp_umem *umem, u64 addr)
 {
 	return NULL;
@@ -137,6 +192,20 @@ static inline dma_addr_t xdp_umem_get_dma(struct xdp_umem *umem, u64 addr)
 {
 	return 0;
 }
+
+static inline u64 *xsk_umem_peek_addr_rq(struct xdp_umem *umem, u64 *addr)
+{
+	return NULL;
+}
+
+static inline void xsk_umem_discard_addr_rq(struct xdp_umem *umem)
+{
+}
+
+static inline void xsk_umem_fq_reuse(struct xdp_umem *umem, u64 addr)
+{
+}
+
 #endif /* CONFIG_XDP_SOCKETS */
 
 #endif /* _LINUX_XDP_SOCK_H */

net/xdp/xdp_umem.c

@@ -165,6 +165,8 @@ static void xdp_umem_release(struct xdp_umem *umem)
 		umem->cq = NULL;
 	}
 
+	xsk_reuseq_destroy(umem);
+
 	xdp_umem_unpin_pages(umem);
 
 	task = get_pid_task(umem->pid, PIDTYPE_PID);

net/xdp/xsk_queue.c

@@ -3,7 +3,9 @@
  * Copyright(c) 2018 Intel Corporation.
  */
 
+#include <linux/log2.h>
 #include <linux/slab.h>
+#include <linux/overflow.h>
 
 #include "xsk_queue.h"
 
@@ -62,3 +64,56 @@ void xskq_destroy(struct xsk_queue *q)
 	page_frag_free(q->ring);
 	kfree(q);
 }
+
+struct xdp_umem_fq_reuse *xsk_reuseq_prepare(u32 nentries)
+{
+	struct xdp_umem_fq_reuse *newq;
+
+	/* Check for overflow */
+	if (nentries > (u32)roundup_pow_of_two(nentries))
+		return NULL;
+	nentries = roundup_pow_of_two(nentries);
+
+	newq = kvmalloc(struct_size(newq, handles, nentries), GFP_KERNEL);
+	if (!newq)
+		return NULL;
+	memset(newq, 0, offsetof(typeof(*newq), handles));
+
+	newq->nentries = nentries;
+	return newq;
+}
+EXPORT_SYMBOL_GPL(xsk_reuseq_prepare);
+
+struct xdp_umem_fq_reuse *xsk_reuseq_swap(struct xdp_umem *umem,
+					  struct xdp_umem_fq_reuse *newq)
+{
+	struct xdp_umem_fq_reuse *oldq = umem->fq_reuse;
+
+	if (!oldq) {
+		umem->fq_reuse = newq;
+		return NULL;
+	}
+
+	if (newq->nentries < oldq->length)
+		return newq;
+
+	memcpy(newq->handles, oldq->handles,
+	       array_size(oldq->length, sizeof(u64)));
+	newq->length = oldq->length;
+
+	umem->fq_reuse = newq;
+	return oldq;
+}
+EXPORT_SYMBOL_GPL(xsk_reuseq_swap);
+
+void xsk_reuseq_free(struct xdp_umem_fq_reuse *rq)
+{
+	kvfree(rq);
+}
+EXPORT_SYMBOL_GPL(xsk_reuseq_free);
+
+void xsk_reuseq_destroy(struct xdp_umem *umem)
+{
+	xsk_reuseq_free(umem->fq_reuse);
+	umem->fq_reuse = NULL;
+}

net/xdp/xsk_queue.h

@@ -258,4 +258,7 @@ void xskq_set_umem(struct xsk_queue *q, u64 size, u64 chunk_mask);
 struct xsk_queue *xskq_create(u32 nentries, bool umem_queue);
 void xskq_destroy(struct xsk_queue *q_ops);
 
+/* Executed by the core when the entire UMEM gets freed */
+void xsk_reuseq_destroy(struct xdp_umem *umem);
+
 #endif /* _LINUX_XSK_QUEUE_H */